Molded plastic containers, more specifically, containers molded from expanded organic polymer material are available in numerous shapes as packaging material for a variety of articles such as eggs, take-out food, light bulbs, seedlings and other articles which one desires to protect against impact, temperature changes and other adverse environmental effects. A large proportion of the containers designed for such purposes are provided with closure means which must withstand rough and rapid handling, or repeated opening and closing. A simple and effective closure is obtained by providing a latch member extending outwardly from or adjacent one edge of one half of the container and which is insertable into a latch opening provided in the mating container half. The latch member may be in the form of a flat tab or round protuberance, or the like. The opening can be provided in the container and the latch member disposed on the lid, or vice versa.
In conventional molding, the operations required to form a latch opening do not lend themselves to the continuous formation of containers. In the usual method of forming a container of expanded polymer material, a wide, continuous sheet of the polymer material is heated to soften it for molding, and the sheet is advanced between the parts of a multi-cavity mold. The mold parts are pressed together to form the containers while they are cooled to fix the polymer material in its molded shape, whereupon the mold parts are separated and the process is repeated indefinitely.
While it is a simple matter to design the mold die surfaces so that a portion of the sheet material is punched out during mold formation to form latch openings, a vacuum cleaning system is required for such operation and is often inadequate for removing all waste material. Such material is often left over in the mold cavity and then is entrained in the outer surface of the next container molded in that cavity, marring that container in appearance by the presence of the waste material embedded in its surface. As a result, an inordinate number of rejects is experienced when attempts are made to form openings in the same operation that molds the container. Accordingly, a subsequent punching step is utilized. However, such a step necessitates feeding the molded container from the molding station to a punching station and registration with punching equipment that includes male and female dies actuated to punch out the openings at the desired location. The slugs of punched out material must then be vacuumed away and the containers fed to a cutting and trimming station for cutting, separation from the large sheets and stacking. The hole punching equipment adds appreciably to the cost and space required for the basic equipment. Precisely adjusted equipment is required for registering the containers accurately with the hole punching equipment so that the openings are properly spaced. Hole punching equipment causes many problems and a significant percentage of containers must be scrapped because of various malfunctions of this accessory. Similarly, the cutting and trimming station adds to the cost of the basic machinery and requires additional precision registration equipment.
A method described and claimed in U.S. Pat. No. 3,845,187 of Oct. 29, 1974 attempts to overcome the foregoing disadvantages by obliquely pressing a hole former through the sheet of plastic during the molding operation so as to provide an opening therein, and by scraping the detached material into a compressed wall adjacent the lower edge of the opening. In the process, the former is in contact with the die surface on which the sheet is being stretched as the molds close. However, there are limitations in such a method in the design of the mold components and in the range of utility of the apparatus. The requirement that the hole former be in contact with the die surface when it cuts through the plastic sheet necessitates frequent sharpening or replacement of the dulled or blunted cutting edge.
The present invention overcomes the disadvantages of prior art without the limitations of the aforementioned patent. Specifically, it provides molding equipment and a method enabling the ready formation of a container by combining the various steps of molding, latch opening forming, reinforcing and cutting, in a single operation.
The container to be molded is from a sheet of expanded organic polymer material, such as expanded polystyrene, and includes latch openings on mating halves which form part of the closure system. The container includes bottom and top parts which may be separate or may be integrally formed with a hinge connection. The parts may be identical or one part, usually the bottom, may be deeper, so that the mating part is considered a lid. Regardless of the function of a container part, the present invention is concerned only with providing an aperture, serving as a latch opening, through one or the other of the parts.
The aperture serving as a latch opening is formed by separating a portion of the sheet from an adjacent region. The separated portion is removed from the area of the opening and used to strengthen a predetermined region adjacent thereto, either by vertical build-up of the adjacent region or by providing a reinforced lip of extended surface. Shearing and removal of the sheared sheet is accomplished by the vertical movement of a shear bar past a vertically aligned die face during compression of the molds. The shear bar entrains the sheared-off sheet portion and moves it downwardly into a cavity of predetermined shape where it is compressed and thereby strengthens particularly vulnerable zones in the vicinity of the latch opening.
The mold utilized to provide the present structure includes male and female mold parts having their die surfaces in confronting relation and shaped to contour the surfaces of the sheet to form the container. The mold parts are spaced from one another a distance sufficient to permit the insertion of a sheet of organic polymer material between the die surfaces. The mold parts are formed with confronting regions for defining projections such as a latch tab or a latch protuberance or ledge at appropriate portions of the container wall. The container is cut and trimmed during the molding operation by means of male and female trim dies disposed on opposite sides of a stripper plate which serves to eject the finished container from the molding station.
A male shear insert which is releasably attached to the interior of the male mold includes a portion which protrudes therefrom through an opening provided in the region of the latch opening. The protruding portion has a vertically oriented flat end face which forms a backwall for a portion of the sheet deployed between the male and female mold parts. A vertically reciprocable female shear bar is attached to the female trim die and includes a flat surface portion which, during the downward travel of the mold, slides past the parallel flat face of the male shear insert at a distance which is less than the diameter of the sheet. Sheet portions which adhere to the vertical face of the stationary male shear insert are thus sheared off by the movable shear bar and carried along with it in a downward direction, to reinforce or build-up a region of the container adjacent the opening. The mold surfaces can be formed so that shearing results in a horizontal extension of the container material to form a reinforced lip, with or without vertical build-up, as will be illustrated in the first and third embodiments to be described in detail. In the second embodiment to be described, the vertical shearing results in a vertical build-up to enable the container to withstand repeated opening and closing operations.